One type of device for analyzing components contained in a liquid sample is the liquid chromatograph. In a liquid chromatograph, a liquid sample is carried by a stream of mobile phase and introduced into a column. The components in the sample are temporally separated within the column and subsequently detected with a detector, such as an absorptiometer, to create a chromatogram. Each component is identified from the position of a peak on the chromatogram, and the concentration of that component is determined from the height or area of that peak (for example, see Patent Literature 1).
In general, a chromatogram obtained by a measurement can be separated into three components, i.e. the peak component, baseline component and noise component. The magnitude of the peak component changes with the elution of various components contained in the liquid sample, while that of the baseline component changes due to such factors as a change in the pressure of the pump supplying the mobile phase or a change in the ambient temperature (baseline drift). The change of the baseline component is normally slower than that of the peak component or noise component. By comparison, the magnitude of the noise component (noise level) fluctuates due to various factors, and its fluctuating amplitude is considerably high. Therefore, it is difficult to isolate the noise component from the chromatogram.
FIG. 1 shows one example of the waveforms (profiles) of a peak component (a), noise component (b) and their sum (c) as well as the power spectra respectively obtained by Fourier-transforming those profiles. As can be seen in those power spectra, the peak component is localized within a low frequency band, while the noise component is spread over a wide frequency band.
Accordingly, in order to extract the noise component from a chromatogram, high-pass filters which reduce frequency components lower than a predetermined frequency have been used. By the filter, the peak component localized within the low frequency band can be mostly removed. As the high-pass filter, for example, a second-order difference filter is used. Using a second-order difference filter as the high-pass filter yields a signal having a similar shape to a profile obtained by the second-order differentiation of the chromatogram. Though not shown in FIG. 1, the baseline component is also normally localized within the low frequency band and therefore can be mostly removed by the high-pass filter, as with the peak component.